1. Field of the Invention
The present invention relates to an LSI package provided with an interface module and a method of mounting the same, particularly, to an LSI package provided with an interface module for transmitting a signal at a high-speed between the external wiring and the interface module and a method of mounting the same.
2. Description of the Related Art
In recent years, the clock frequency of an LSI is being made higher and higher, and a CPU for a personal computer that is operated under a clock frequency of the order of GHz has been put to a practical use. However, the pace of the improvement in the throughput at the interface between the adjacent LSIs is moderate, compared with the increase in the clock frequency, which constitutes a bottle neck in the performance of the personal computer. Such being the situation, the research and development for improving the throughput in the interface are being conducted vigorously.
For improving the throughput of the interface, it is necessary to increase the signal frequency per terminal and to increase the number of terminals. However, the increase in the number of terminals is limited because, if the number of terminals is increased, the areas of the LSI and the package thereof are enlarged so as to increase the length of the internal wiring, with the result that it is impossible to operate the LSI under a high frequency. It follows that it is of high importance to increase the frequency per terminal. On the other hand, if the frequency per terminal is increased, the attenuation of the electric signal is increased so as to increase the influence on the reflection produced by the impedance mismatch. Such being the situation, the line length is limited. Under the circumstances, it is necessary to use a transmission line that permits greatly suppressing the impedance mismatch and the attenuation amount as a high-speed signal transmission line.
It is effective to use an optical fiber as a long-distance transmission line that is small in the influences produced by the impedance mismatch and by the loss. Therefore, an optical interface module performing the photo-electric converting function is used as the interface module. The interface modules commercialized by using the optical interface module include, for example, the transceiver module disclosed in “Proceedings. 51st Electronic Components and Technology Conference, P.P. 880-5,2001”.
In the transceiver module disclosed in the literature quoted above, an LSI for processing signals is incorporated in a PGA (programmable gate array) package. This PGA package is mounted to a mounting board. The input-output signal from the LSI is transmitted through the package into the optical interface module that is mounted to the mounting board and is further transmitted from the optical interface module into a signal line. The optical interface module includes optical elements such as a semiconductor laser element (LD) and a photo-detecting element (PD) as well as an optical fiber, and an optical signal is received from and transmitted to an external circuit through the optical fiber. Also, an interface IC for driving the optical element is housed in the optical interface module so as to be connected to a signal line on the mounting board, to a required control signal line and to a power source line (not shown) through an electrical input-output terminal. Each of the LSI and the optical interface module is provided with a heat sink for the heat dissipation for the cooling.
In the board edge mounting type optical interface module of the configuration described above, the electric signal is converted into an optical signal by the photo-electric converting function so as to permit the converted optical signal to be introduced into the optical fiber. Since the loss is very small and the limitation of the band is small in the optical fiber, it is possible to transmit the signals at a high-speed even if the transmission line is relatively long as in the transmission between mounting boards or between devices. However, in the optical interface module, the electric signal is received and transmitted through the signal line on the mounting board and, thus, the signal transmission is affected by the attenuation of the electric signal on the mounting board or by the impedance mismatch. Since the maximum length of the signal wiring of the mounting board exceeds 30 cm, a highly expensive transmission line is required for transmitting signals having a high frequency, e.g., a signal of 10 Gbps, so as to give rise to the problem that the cost of the mounting board is increased.
Under the circumstances, an improved technology for the signal transmission at a high-speed is proposed in, for example, “HOT9 Interconnects, Symposium on High Performance Interconnects, P.P. 31-5,2001” and “Nikkei Electronics No. 810, Dec. 3, 2001, pp. 121-122”. Specifically, it is proposed that signals are transmitted only within an interposer of an LSI package without using a mounting board so as to shorten the electrical wiring as much as possible, and the electric signal is converted into an optical signal on the interposer for receiving the signals from the external device or for transmitting the signals to the external device.
Each of the publications quoted above discloses the configuration that an optical interface module is fixed by welding to the interposer of the LSI, and the interposer of the LSI is optically connected to the optical interface module by a fiber comprising an optical connector.
In this configuration, the LSI for the signal processing is electrically connected to the interposer by a solder bump. An optical interface module is mounted to the interposer by a solder bump. The input-output terminals of the LSI are connected to a wiring, and the wiring is connected to the optical interface module. An interface IC and an optical element are housed in the optical interface module, and the electric signal is converted into an optical signal by the interface IC and the optical element. The interface IC and the optical element are housed in a package provided with an input-output window for the optical signal so as to ensure the reliability as the optical interface module.
A flat micro-lens plate is mounted to the input-output window so as to permit the light beam incident on the optical interface module and the light beam emitted from the optical interface module to be converged by the micro lens. The micro lens imparts a big tolerance relative to the optical coupling with the optical fiber mounted on the outside. The interposer is electrically connected to the solder bump by the mounting board. One end of the optical fiber is connected to an optical connector comprising a mirror for changing the optical path by 90°. An aligning pin mounted to the optical connector is inserted into a coupling hole of the package so as to determine the positions of the optical connector and the package so as to permit the micro lens and the optical fiber to be aligned.
According to the configuration described above, the optical interface module is mounted to the interposer after the interface IC, the optical element, etc. have been packaged. Therefore, the optical interface modules are individually inspected so as to make it possible to mount a good optical interface alone having a high reliability, thereby suppressing the inspection cost. Also, since the optical connector is connected after the interposer is mounted to the mounting board, advantages in the manufacturing process can be obtained. For example, it is unnecessary to take into consideration the deterioration of the resin cover caused by the heat treatment in the mounting stage of the interposer and other parts. It is also unnecessary to consider the limitation in the handling of the optical fiber such as the bending leading to the breakage.
However, the particular configuration requires the soldering of the LSI to the interposer, the soldering of the optical interface module to the interposer, or the soldering of the interposer to the mounting board. It should be noted in this connection that the LSI package must be assembled by changing the melting points of the solders such that a certain soldering does not cause a defect in the other soldering. Also, the mounting procedure is limited in assembling the parts of the LSI package. Further, in order to hold the optical connector, required is a mechanism for pushing the optical connector to the package so as to hold the optical connector, and the mechanism of the apparatus tends to be rendered bulky in the case where the optical connection is achieved by using a connector. Still further, if a holding mechanism is mounted to the apparatus, the space in which is arranged a heat sink mounted to an upper portion of the LSI is limited so as to render complex the configuration and to increase the cost. It follows that it is difficult to mount the heat dissipating heat sink of the optical interface module.
In general, the power consumption per terminal tends to be increased with increase in the transfer frequency of the signal. For example, the power consumption of some of the LSIs has come to reach 70 to 80 W in recent years in the CPU used in a personal computer. Therefore, the apparatus is constructed such that a heat spreader and a gigantic heat sink are mounted on the signal processing LSI so as to ensure a large heat dissipating area, and a compulsory air cooling is performed by using, for example, a fan. On the other hand, it is necessary to decrease the wiring length between the signal processing LSI and the interface module as much as possible as described previously. Therefore, in the case of mounting a heat sink for the signal processing LSI, there is no allowance in the space for mounting another heat sink for the interface module.
Under the circumstances, it is conceivable to mount a heat sink shared by the signal processing LSI and the interface module so as to achieve the heat dissipation simultaneously from the signal processing LSI and the interface module. However, where the signal processing LSI and the interface module are mounted simultaneously to the interposer 2, it is difficult to align strictly the upper surfaces of the signal processing LSI and the interface module and to set the difference in level strictly at a prescribed value.
What should also be noted is that, since the interface module is soldered, it is also necessary to renew the expensive signal processing LSI in the event of the disorder of the interface module.
The configuration that an optical element is mounted directly to the interposer 2 and an optical waveguide made of an organic material is attached to the mounting board so as to form a transmission line is disclosed in “16th Academic Lecture Meeting of Electronics Mounting, 20B-10, 2002”.
In the particular configuration, an interface IC is soldered to the interposer. The interposer is fixed to a mounting board with a spacer interposed therebetween. The mounting board and the interposer are connected to each other by, for example, a flexible wiring, and a power source, an input-output electric signal, etc. are supplied to the mounting board and the interposer. In this configuration, it is assumed that the signal processing LSI, etc. are mounted in the three dimensional direction above the interface IC.
A surface-emitting type optical element is mounted to the interposer on the side of the mounting board, and the positions of an optical waveguide and an optical element are determined to permit the optical element to be optically coupled with the optical waveguide including a mirror mounted to the mounting board for changing the optical path by 90°. Also, an electrode is mounted to extend through the interposer so as to decrease the length of the wiring for the electric signal, thereby obtaining good signal characteristics.
In the particular configuration, the optical element as a bare chip is mounted directly to the interposer. When the interposer is mounted to the mounting board, the optical element is optically coupled with the optical waveguide. It follows that it is difficult to maintain an optical accuracy because of the difference in the thermal expansion coefficient between the mounting board and the interposer. Also, if the optical element is mounted as a bare chip, it is difficult to ensure the reliability of the optical element. In order to ensure the reliability, it is necessary for the optical element portion to be buried in, for example, the resin transparent to the wavelength used for the signal transmission so as to require a processing operation on the mounting board. It follows that much limitation is imposed on the manufacturing process so as to increase the manufacturing cost. Further, since it is necessary to attach separately the optical waveguide to the mounting board, the mounting process is rendered complex so as to increase the mounting cost. An additional problem to be noted is that, in the particular configuration, it is necessary to renew the optical element together with the expensive signal processing LSI in the event of the disorder of the optical element.
The problems described above in conjunction with the prior art using an optical fiber as a transmission line are also brought about in the case of using an electrical transmission line such as a coaxial cable, a semi-rigid cable or a flexible wiring board.
As described above, various optical interface modules are used for improving the throughput of the conventional interface. However, the board edge mounting type optical interface module disclosed in “Proceedings. 51st Electronic Components and Technology Conference, P.P. 880-5,2001” gives rise to the problem that a costly transmission line is required for transmitting signals having a high frequency so as to increase the cost of the mounting board.
Also, the configuration disclosed in “HOT9 Interconnects. Symposium on High Performance Interconnects, P.P. 31-5,2001” and “Nikkei Electronics No. 810, Dec. 3, 2001, pp/121-122” gives rise to the problems in the mounting that the mechanism is rendered excessively bulky because a connector system is employed in the configuration, and that careful attentions are required for the soldering. Further, the configuration is rendered complex because it is necessary to ensure the space for mounting a heat sink so as to give rise to the problems that the manufacturing cost is increased and that it is difficult to mount a heat sink for the heat dissipation from the optical interface module. Where the heat sink is shared by the signal processing LSI and the interface module, it is difficult to allow the upper surfaces of the signal processing LSI and the interface module to be aligned strictly and to control the difference in level at a prescribed value strictly in mounting simultaneously the LSI and the interface module to the interposer. Further, since the interface module is soldered, it is also necessary to renew the expensive signal processing LSI in the event of the disorder of the interface module.
Further, the configuration disclosed in “16th Academic Lecture Meeting of Electronics Mounting, 20B-10, 2002” gives rise to the problem that it is difficult to maintain an optical accuracy because of the difference in the thermal expansion coefficient between the mounting board and the interposer. Further, since it is necessary to mount separately an optical waveguide to the mounting board, the mounting process is rendered complex so as to increase the mounting cost. In addition, in the event of the disorder of the optical element, it is necessary to renew the expensive signal processing LSI in the event of the disorder of the optical element.
The problems similar to those described above are also brought about in the configuration in which is used an electric interface module that does not involve an optical element.